Loudspeaker having a conical diaphragm



A ril 19, 1955 F. H. BRITTAIN 2,706.529 LOUDSPEAKER HAVING A CONICAL DIAPHRAGM Filed Sept. 29. 1952 5o 100 250 500 1,600 2 00 5,000 40,000 c/s db FIG. HA) +20.

250 500 1,000 2,500 5,000 10,0CXJ C/S FIG. \(b) Q: 7 nun INVENTOR EHMC/g GH FTWTTHN HTTQ'RNE United States Patent LOUDSPEAKER HAVING A CONICAL DIAPHRAGM Francis Hugh Brittain, Pinner, England, assignor to The General Electric Company Limited, London, England, a British company This invention relates to loudspeakers having a conical diaphragm. It is to be understood that in this specification the term conical includes frustoconical and the term cone includes a frustum of a cone.

In such loudspeakers it has been found that an undesirable dip in the frequency response curve of the loudspeaker may occur at a frequency within its normal frequency range. This effect is particularly noticeable with loudspeakers having conical metal diaphragms, but may also occur in loudspeakers having conical diaphragms of other materials such as paper.

It is an object of the present invention to provide a loudspeaker having a conical diaphragm in which such an undesirable dip in the frequency response curve of the loudspeaker is substantially eliminated.

According to the invention, a loudspeaker having a conical diaphragm is provided with a rigid plug disposed within the space bounded by the diaphragm and spaced from the diaphragm, the form and size of the plug and its position relative to the diaphragm being such that a dip in the frequency response curve of the loudspeaker, which would occur at a frequency within its normal frequency range in the absence of the plug, is substantially eliminated.

According to one aspect of the invention, a loudspeaker having a conical diaphragm is provided with a rigid plug disposed within the space bounded by the diaphragm and spaced from the diaphragm, the plug extending from the vicinity of the apex of the diaphragm and occupying at least 10% of the space bounded by the diaphragm, and at least that portion of the plug which is nearest the apex of the diaphragm having a surface disposed close to the diaphragm. Preferably, the plug occupies not more than 20% of the space bounded by the diaphragm. Thus at least a portion of the plug may be in the form of a cone which extends from the vicinity of the apex of the diaphragm coaxial with the diaphragm and with its curved surface disposed close to the diaphragm, the cone having an axial length which is at least 15%, and is preferably not more than 25%, of the axial length of the diaphragm.

One arrangement in accordance with the present invention will now be described by way of example, with reference to the accompanying drawings in which,

Figures 1(a) and 1(b) are explanatory diagrams, and

Figure 2 is a sectional view of part of a 6" moving coil loudspeaker having a conical metal diaphragm.

It has been found that a 6" moving coil loudspeaker having a conical metal diaphragm and of a conventional construction has a frequency response curve similar to that shown in Figure 1(a) of the accompanying drawings, in which frequency in cycles/second is plotted as abscissa and the relative response of the loudspeaker in decibels is plotted as ordinate. As will be seen from Figure 1(a), a dip in the frequency response curve occurs at a frequency of about 2.5 kilocycles/second. In order to eliminate this dip in the frequency response curve, the construction shown in Figure 2 of the drawings is adopted for the loudspeaker. Referring to Figure 2, the loudspeaker includes a right circular conical metal diaphragm 1 having an axial length of 1 /2" and a conical angle of 115, the diaphragm 1 being welded to a former 2 carrying the voice coil 3, which is mounted in a gap in the magnetic circuit 4. A rigid plug 5 is secured by means of a screw 6 to the part 7 of the mag netic circuit disposed inside the former 2 so as to be 2,706,529 Patented Apr. 19, 1955 ice mounted within the space bounded by the diaphragm The plug 5 is in the shape of two coaxial circular cylinders 8 and 9 joined by a right circular conical part 10. The outer cylinder 8 has a diameter of 1%" and a length of while the inner cylinder 9 has a diameter of Va" and a length of /8". The conical part 10 has an axial length of @i and its conical angle is the same as that of the diaphragm 1, the plug 5 being mounted in position so that the surface of the part 10 is parallel to the internal surface of the diaphragm 1 and these surfaces are spaced apart. With the construction described above and shown in Figure 2, the loudspeaker has a frequency response curve similar to that shown in Figure 1 (b), and it will be seen that the undesirable dip in the frequency response curve has been substantially eliminated.

The spacing between the surface of the conical part 10 and the diaphragm 1 is fairly critical, since if it is made too large (say more than A") the dip in the frequency response curve will not be eliminated, while if it is made too small (say less than $5 the plug 5 may be struck by the diaphragm 1 when the loudspeaker is reproducing sounds of low frequency. In order to ensure that the dip in the frequency response curve is eliminated, it is also necessary that the axial length of the conical part 10 should be greater than a certain minimum value, which is approximately 15% of the axial length of the diaphragm 1. Furthermore, it is also desirable that the axial length of the conical part 10 should be below a certain maximum value, which is approximately 25% of the axial length of the diaphragm 1, since if it is made too long the response of the loudspeaker at frequencies of the order of 5 to 6 kilocycles/second will be impaired. The limits on the axial length of the conical part 10 and the spacing of its surface from the diaphragm 1 will also impose limits on the diameter of the cylindrical part 8. With a given diameter for the part 8, its length must not be too small or the dip in the frequency response curve will not be eliminated, While it is desirable that its length should not be too great, otherwise the high frequency response of the loudspeaker will again be impaired. With a conical part 10 of the dimensions described above, the length of the cylindrical part 8 may suitably be made approximately 50% of the axial length of the diaphragm 1.

It will be appreciated that plugs of other shapes than that described above may be used. However, it is necessary in each case that the plug should occupy at least 10% of the space bounded by the diaphragm in order to eliminate the undesirable dip in the frequency response curve of the loudspeaker, while it is desirable that the plug should occupy not more than 20% of the space bounded by the diaphragm in order that the high frequency response of the loudspeaker should not be impaired. The plug may be made of any convenient material, for example wood, metal or one of the materials commonly known as plastics, since its function appears to be merely to fill up part of the space bounded by the diaphragm. For the same reason the plug may also be made hollow if desired, provided that in this case the walls must not be so thin that the plug is no longer rigid.

I claim:

1. A loudspeaker having a conical diaphragm and provided with a rigid plug disposed within the space bounded by the diaphragm and spaced from the diaphragm, the plug occupying at least 10% of the space bounded by the diaphragm, and at least a portion of the plug being in the form of a cone which extends from the vicinity of the apex of the diaphragm coaxial with the diaphragm and with its curved surface disposed close to the diaphragm, the cone having the same conical angle as the diaphragm and having an axial length which lies between 15% and 25% of the axial length of the diaphragm.

2. A loudspeaker as set forth in claim 1, in which the plug occupies not more than 20% of the space bounded by the diaphragm.

3. A loudspeaker as set forth in claim 1, in which the plug includes a portion in the form of a cylinder one end of which coincides with the outer end of the cone, the cylinder being disposed coaxial with the diaphragm and having an axial length equal to approximately 50% of the axial length of the diaphragm.

4. A loudspeaker as set forth in claim 1, in which the spacing between the diaphragm and the curved surface of the cone lies within the range of to A.

5. A loudspeaker as set forth in claim 1, in which the diaphragm is of metal.

References Cited in the file of this patent UNITED STATES PATENTS Frederick July 19, Fisher May 3, Tiedje July 26, Scott I an. 27, 

